Device for measuring light incident on an image forming optical system

ABSTRACT

The present invention relates to a device for measuring light incident on an image forming optical system, making use of a plural number of the diffraction element. The plural number of the diffraction elements are respectively provided in the optical path of the image forming optical system. Further the plural number of the diffraction elements respectively present a certain predetermined area. The plural number of the diffraction elements, seen along the direction of the optical axis of the image forming optical system, are substantially only partially overlapped with respect to each other. The light beam incident on a diffraction element is divided into a non diffracted light beam and a diffracted light beam. Thus diffracted light beam is directed toward the light detecting means so as to be measured while the non diffracted light beam advances along the above mentioned optical axis. Consequently at the part at which the diffraction elements overlap each other, the non diffracted light beam led out of the preceeding diffraction element again enters the following diffraction element. This light beam is again divided into a non diffracted light beam and a diffracted light beam by means of the following diffraction element. The diffracted light beam is measured by a light detecting means in the same way as in the above mentioned case, while the non diffracted light beam advances along the above mentioned optical axis. Thus the light beam which passes through the overlapping parts is measured a plural number of times.

BACKGROUND OF THE INVENTION

The present invention relates to a device for measuring light incidenton an image forming optical system making use of a plural number ofdiffraction elements.

A patent application for a light measuring method by means ofdiffraction elements has been applied to patent by the present inventorsin the U.S.A. on Dec. 6, 1973 under application Ser. No. 422,337 and inWest-Germany on Dec. 11, 1973 under application No. P 2361626.4.

The device in accordance with the present invention is an improvement ofthe above mentioned applications, being improved in such a manner that acertain determined area of the light beam can be measured with weight.The present improvement is made when the device in accordance with theabove mentioned application is applied to a single reflex camera as adevice to carry out a light measurement, putting weight on the lightmeasurement in the central part. The light measurement with weight inthe central part means a light measuring method by means of which theweight is put on the light measurement move in the central part of thepicture field than in the circumferential part. This light measurementwith weight in the central part is carried out normally in such a mannerthat the output of the light detecting means sensing only the light beamcoming from the central part of the picture field is added to the outputof the light detecting means sensing the light beam coming from thewhole picture field.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a device capable ofmeasuring a light beam in a certain predetermined area with priority.

The above mentioned purpose can be realized by arranging in the opticalpath of the image forming optical system only partially overlappingdiffraction elements with substantially different area in such a mannerthat the light beam incident on the overlapping part is diffracted aplural number of times to be measured.

Hereby the word "substantially different area" is to be understood toinclude the case the areas are equal, because in case diffractionelements are provided sufficiently apart from each other in theconverging light beam from the image forming optical system, the areasfor light measurement are different even if the diffraction elementspresent equal areas.

Further the diffraction elements to be applied to the device inaccordance with the present invention present gratings which can bedistinguished from other parts (normally of gelatine) due to thedifference in the density or refraction index. Hereby the gratings caneither be of planar type or of volume type with thickness in threedimensional directions. However taking the loss in the light amount intoconsideration, the diffraction element of phase (different only inrefraction index) volume type is preferable. Experiments have provedthat the best result can be obtained with the thickness 1 - 20 μm.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 shows the first embodiment of the light measuring device inaccordance with the present invention.

FIGS. 2 to 7 respectively show the composition of the diffractionelement to be applied to the second to the seventh embodiment of thelight measuring device in accordance with the present invention.

FIGS. 8 and 9 respectively show a manufacturing method of thediffraction elements to be applied to the light measuring device inaccordance with the present invention.

FIG. 10 shows a variation of the manufacturing method shown in FIG. 9.

FIGS. 11 to 14 respectively show a drawing for explaining the case thelight measuring device in accordance with the present invention isapplied to a single reflex camera.

FIGS. 15 and 16 respectively the color dispersion characteristics of thedeffraction elements.

FIGS. 17, 18 and 19 respectively show the eighth, the nineth and the10th embodiments, whereby the color dispersion characteristics of thediffraction element is compensated.

FIG. 20 shows a drawing for explaining the light beam diffracted againby means of a diffraction element.

FIG. 21 shows the 11th embodiment of the diffraction element, whereby nolight beam is diffracted again.

FIG. 22 shows a drawing for explaining the problem as to the limitationof the size of the diffraction gratings peculiar to the diffractionelement, by means of which a diffracted light beam of self convergenttype can be obtained.

FIGS. 23 and 24 respectively show the 12th embodiment, having solved theproblem shown in FIG. 22.

FIG. 25 shows a veriation of the twelfth embodiment.

FIG. 26 shows a drawing for explaining the polarization characteristicsof the diffraction element.

FIGS. 27, 28 and 29 respectively show a drawing for explaining the 13thembodiment, freed from the influence by the polarization characteristicsof the diffraction element.

FIGS. 30, 31 and 32 respectively show a drawing for explaining the 14thembodiment as variation of the thirteenth embodiment.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

Below the present invention will be explained in accordance with theembodiments of the present invention shown in the accompanying drawings.In the following embodiments a plural number of diffraction elements arerepresented by two diffraction elements for the sake of the simplicityof the explanation.

FIG. 1 shows the first embodiment of the device in accordance with thepresent invention. In the drawing, 1 is the image forming optical systemfor forming an image on the image plane. 3 and 4 are respectively adiffraction element with difference surface area, both diffractionelements being arranged in the optical path of the light beam 5 comingfrom the image forming optical system 3, 4. 6 and 7 are respectively alight sensor for sensing the light beams 8 and 9 diffracted by thediffraction elements 3 and 4. In this way the light amount of the lightbeam coming from the image forming optical system is measured by meansof the light sensors 6 and 7. Hereby the light beam 9 diffracted bymeans of the diffraction element is shown as if it were reflected bymeans of the diffraction element 4, whereby in reality it advances alongthe same direction as that of the light beam 8 diffracted by means ofthe diffraction element 3. This is only for the sake of the simplicityof the drawing.

FIG. 2 shows the second embodiment of the device in accordance with thepresent invention. Different from the first embodiment, the diffractedlight beams 8 and 9 are reflected by means of the totally reflectingplane so as to be directed perpendicular to the direction along whichthe light beam 5 advances. Namely the diffraction elements 3 and 4 areheld by means of glass plates 11 and 12, whose refraction index almostequal to that of the diffraction elements, whereby further thediffraction elements 3 and 4 are so designed that the diffracted lightbeams 8 and 9 are totally reflected on the surface of the glass plates11 and 12.

FIG. 3 shows the third embodiment of the device in accordance with thepresent invention. In case of the second embodiment the boundary planebetween the diffraction elements 3 and 4 and the air is made use of asthe totally reflecting plane while in case of the third embodiment thelower plane of the cover glasses 13 and 14 presenting almost the samerefraction index as that of the diffraction elements 3 and 4 and beingin close contact therewith are made use of as the totally reflectingplane.

Further as in case of the fourth embodiment shown in FIG. 4. Twodiffraction elements 3 and 4 can be cemented on one glass plate 11.

Further as in case of the fifth embodiment shown in FIG. 5 twodiffraction elements 3 and 4 can be provided between two glass plates 11and 13.

Further as is shown in FIGS. 6 and 7 two diffraction elements 3 and 4can be brought in close contact with each other, without forming an airgap between the two diffraction elements 3 and 4 as in case of thesecond and the third embodiment. In case of the so far mentionedembodiments the diffraction elements are so designed that the diffractedlight beams 3 and 4 advance to the right and the left respectively,while they can also be so designed that both diffracted light beams 3and 4 advance along the same direction. Further in case of the so farmentioned embodiments two diffraction element are used, while two kindsof diffraction elements can be used, being held over each other on oneholder.

Such diffraction elements can be manufactured by engraving lines orinclined grooves for example on glass plates, whereby the easiest methodfor manufacturing the elements is shown in FIG. 8. Below themanufacturing method will be explained in accordance with the case theone diffraction element 3 shown in FIG. 2 is manufactured. 15 is thesource of a coherent light such as laser tube. 16 is the beam expander,17 the beam splitter, 18 and 19 the coherent light beam splits into twobeams by means of the beam splitter 17 and 20 the reflecting mirror.

21 and 22 are the prisms with almost same refraction index. Betweenthere prisms 21 and 22 a photo-sensitive agent coated on a glass plate23 is inserted. Hereby it is desirable that the glass plate 23 can laterbe made use of as the afore mentioned plane parallel glass plate 11,whereby it refraction index is almost equal to that of the prisms 21 and22. Further it is desirable that the photosensitive agent 24 is coatedthick enough for forming a volume hologram while the photo-sensitiveagent capable of forming a phase hologram such as chromated gelatine orphotopolymer is preferred. Further in order to avoid the formation of anair gap between the prisms 21 and 22, it is desired that a liquid withalmost the same refraction index as that of the optical glass be putbetween the two prisms 21 and 22.

The one coherent light beam 18 split by means of the beam splitter 17 isdirected so as to penetrate the glass plate 23 while the other coherentlight beam 19 is directed by means of the mirror 20 toward the glassplate with such an angle that the coherent light beam 19 is totallyreflected on the upper plane of the glass plate 23. In this way, aholographic diffraction element is manufactured.

FIG. 9 shows other manufacturing method of diffraction element than whatis shown in FIG. 8. The difference from the method shown in FIG. 8 isthat in the optical path of the coherent light beam 19 a cylindricallens 25 is provided. As is shown in FIG. 10 the light beam 19 isconverged at the position at which the light sensing element 7 is to beprovided later and then diverged. Consequently the manufacturing methodshown in FIG. 9 is profitable when a diffraction element manufactured bymaking use of such a cylindrical lens is applied to the device inaccordance with the present invention, because the diffracted light beamcan be converged on the light sensor without any condenser lens. Hereby26 is a light absorbing material.

As explained so far the device in accordance with the present inventionoffers various profits because a plural number of the diffractionelements are provided in the light beam coming from the objective lens.For example, by arranging on the focus plane of the objective lens ofthe single reflex camera the device so designed that the surface area ofthe two diffraction elements can be varied, the central part of the twodiffraction elements can be laid over each other or the external part ofthe two diffraction elements can be varied, the mean light measurementwith weight in the central part is made possible in the TTL lightmeasurement or many diffracted light beam can be led to the light sensorwithout causing a change of the color tone on the finder picture plane.This point will be explained below.

An embodiment of the device in accordance with the present invention,being applied to the single reflex camera will be explained inaccordance with FIG. 11.

30 is the photographic lens, 31 the aperture, 32 the quick returnmirror, 33 the image forming plane, 34 the shutter, 35 the focus glass(matted plate), 36 the diffraction element, 37 the condenser lens, 38the pentagonal prism, 39 the eye piece, 40 the eye and 41 the lightdetecting means. The light beam coming from outside is focussed on thefocus plane by means of the photographic lens 30, whereby the light beamfrom the focus plane enters into the diffraction element 36 in such amanner that the diffracted light beam reaches the light detecting means41, being totally reflected while the non diffracted light beam reachesthe eye through the condenser lens, the pentagonal prism and the eyepiece.

The composition of the diffraction element is shown more in detail inFIGS. 12, 13 and 14. Hereby FIG. 12 shows the plane view, FIG. 13 theside view and FIG. 14 the front view. As the diffraction element 41 anyof the afore mentioned embodiment can do, while in the followingexplanation the element is represented by what is shown in FIG. 5.Hereby the diffraction element is manufactured by means of the methodshown in FIG. 9 or FIG. 10 so as to be able to obtain a self convergentdiffracted light beam. Further the first diffraction grating 42 of theelement as the circle of a radius r with the center at 4 on the glassplate 43, 44, whereby the diffraction gratings 42 are so designed as toconverge the diffracted light beam 46 by means of the firstphotoelectric converter 47. The second diffraction grating 48 is acircle of a radius R with the center at 45 similarly to the case withthe first diffraction grating 42. Hereby R is larger than r. Furtherthis diffraction grating 48 is so designed as to converge the diffractedlight beam on the second photoelectric converter 49.

In consequence the light beam at the central spot can be measured bymeasuring the light beam the first diffraction grating 42 while the meanlight beam can be measured by measuring the light beam from the seconddiffraction grating 48 and the light beam is measured with the weight atthe center by measuring the composed light beam of the beams from thetwo gratings 42 and 48. Consequently for example by providing shuttermeans (not shown in the drawing) in from of the light sensors 47 and 49in such a manner that the shutter means can selectively closed andopened, the light measurement with weight at center and the mean lightmeasurement can selectively carried out. In case the results of thethree kind of the light measurement are indicated by means of one meterand the diffraction efficiencies of the gratings are equal to eachother, it is necessary to provide filter means because there take placea difference between the light amounts to the light sensors. Namely, itis necessary to provide a filter in front of the light sensor 48 forreducing the light amount in accordance with the ratio of the area ofthe gratings 42 to that of the gratings 48. Further in case of the lightmeasurement with weight at center the outputs of the light sensors 47and 49 are summarized so that it is necessary to reduce each lightamount by half by means of filter means (not shown in the drawing) orthe like. Hereby it suffices to change the sensitivity of the lightsensors without making use of filter means.

So far the general explanation of the device in accordance with thepresent invention has been made and therefore below various improvementconnected thereto will be explained in accordance with severalembodiment.

The fist improvement relates to the elimination of the light measurementerror arising from the difference between the diffraction angles of thediffracted light beams due to the difference in the wave length. Thereason for this light measurement error will be explained in accordancewith FIGS. 15 and 16. As is shown in FIG. 15, the light beam incident onthe diffraction element 60 is different in the diffraction incidenceangle and the diffraction angle due to the wave length in accordancewith the Bragg's diffraction condition. In the drawing, B, G and R arerespectively the light beam with the wave length for blue, green andred. The diffracted light beam are totally reflacted several times onboth planes 61 and 62 of the diffraction element and then led out at theend surface 63 of the diffraction element. The beam diffracted at onepoint of the diffraction element 60 is different in the diffractionangle in accordance with the wave length so that at the end surface 63of the diffraction element the position at which the light beam is ledout is different in accordance with its wave length. The direction ofthe led out light beam is determined in accordance with the wave lengthand the angle made with the plane of the diffraction element increase inthe sequence of R, G and B. Unless all the light beams led out at theend surface of the diffraction element are led to the light sensingelement, there is a danger that there takes place a part which is notmeasured, depending upon the wave length.

FIG. 16 shows the distribution of the light beam along the direction ofthe plane of the diffraction element, at the end surface of thediffraction element at which the light beam is led out, the diffractionelement consisting of the diffraction gratings manufactured inaccordance with the method shown in FIG. 10. The light beam diffractedby means of the diffracting gratings manufactured as mentioned above isconverged at one point. When the diffraction element is so designed thatthe light beam is converged at one point on the end surface 63, thelight beam is led out of the diffraction element as if the beam wereradiated from the point. Hereby only the light beam with the wave lengthused for manufacturing the diffraction gratings is converged at onepoint due to the dispersion of the diffraction gratings, whereby thelight beam with other wave length than the above mentioned is notconverged completely while the point with the highest intensity deviatesfrom the point at which the light beam with the above mentioned wavelength is converged. In case the diffraction gratings are manufacturedin such a manner that the light beam with the wave length for blue isconverged on the end surface of the diffraction element, the point atwhich the intensity of the light beam diffracted by means of thediffraction gratings is highest deviates from the end surface toward theinside of the diffraction element according as the wave length of thelight beam concerned approaches that for red. Hereby the light beam ledout of the diffraction element advances similarly although the positionof the center of the divergence is different. Consequently in case of adiffraction element with large light measuring area, the angle in whichthe light beam led out at the end surface of the diffraction elementbecomes remarkably large. There takes place a danger for the error ofthe light measurement if the light beam making a large angle with thecentral axis does not reach the light sensing element.

FIG. 17 shows the first measures for avoiding the above mentioned errorof the light measurement. The light detecting means P is in closecontact with the end surface of the diffraction element 60. Hereby it isdesired that the size of this light detecting means P be equal to orlarger than the thickness of the end surface 63 and the diffractiongrating be so designed that the diffracted light beam is converged onthe end surface 63. By means of designing the light detecting means asmentioned above, all the light can be directed toward the lightdetecting means P so as to obtain a superior light measurement.

FIG. 18 shows the nineth embodiment as the second measures for avoidingthe error of the light measurement. Namely in case of the embodimentshown in FIG. 18, a reflection cylinder is provided between the endsurface 63 and the light detecting means P. Hereby 60 is the diffractionelement, 64 the reflection cylinder, and P the light sensing element. Areflction cylinder consisting of a cylindrical glass rod with Ag, Al orthe like metalized on the external surface of consisting of a holecylinder with Ag, Al or the like metalized on the internal surface ismounted on the end surface 63 of the diffraction element 60, while thelight sensing element P is provided at the other end of the reflectioncylinder so as to avoid the error of the light measurement.

FIG. 19 shows the third measures. Hereby 60 is the diffraction element,65 the cylindrical lens and P the light sensing element. A cylindricallens with Ag, Al or the like metalized on the upper and the lowersurface 66 and 67 as reflection plane is mounted on the end surface 63of the diffraction element 60 so as to decrease the range of the anglein which the light beams led out of the diffraction element advancealong the horizontal direction of the diffraction element and so as tolead the light beams to the light sensing element efficiently.

Below an embodiment for solving the problem of the repeated diffractionwill be explained. At first the repeated diffraction will be explainedin accordance with FIG. 20. Hereby 70 is the diffraction element. 71 isthe light beam incident on this diffraction element 70, whereby thelight beam is divided by the diffraction element into the diffractedlight beam 72 and the non diffracted light beam 73. As explained above,the diffracted light beam 72 is totally reflected by the totallyreflecting planes 74 and 75, led out at the end surface 76 and directedtoward the light detecting means 77. Hereby the light beam reflected bythe totally reflecting plane 74 and 75 could be diffracted again bymeans of the diffraction gratings 78. In case the diffraction element issufficiently thick, the light beam reflected by means of the totallyreflecting plane 74 to reach the diffraction gratings 78 is neverdiffracted again while the light beam reflected by the totallyreflecting plane 75 to reach the diffraction grating 78 again isdiffracted again. Such repeatedly diffracted light beam 79 becomes aghost light beam, which is very inconvenient.

The first measure to solve this problem of the repeated diffraction isto limit the size of the diffraction gratings so as to prevent therepeated diffraction. Supposing that the distance between the position71 at which the light beam is incident on the diffraction element andthe position 79 at which the light beam is diffracted again be lo, theangle between the diffracted light beam 72 and the normal line be θ andthe distance between the totally reflecting planes 74 and 75 of thediffraction element be d, lo can be expressed by 2·d·tan θ. Thus bymaking the diffraction gratings smaller than 2·d·tan θ, the problem ofthis repeated diffraction can be solved.

The second measures is to change the angle selection characteristics ofthe diffraction gratings at the position at which the repeateddiffraction takes place so as to prevent the repeatedly diffracted lightbeam. In order to change the angle selection characteristics it sufficesto change the inclination along the direction of the thickness of thediffraction gratings or the pitch of the diffraction gratings.

FIG. 21 shows the 11th embodiment as the third measures, whereby bymeans of inclining the one totally reflecting plane in such a mannerthat the inclination of the light beam reaching the diffraction gratingsagain is changed, the repeated diffraction is prevented.

Below the desirable conditions for the diffraction gratings of thediffraction element in case the device in accordance with presentinvention is applied to a single reflex camera will be explained. Atfirst it is desired that the diffraction gratings should correspond withthe spectroscopic sensitivity characteristics of the light detectingmeans to be applied to a camera. Namely in case the sensitivity of thelight detecting means for red is inferior, it is natural that thediffraction grating diffracting the red light beam most be desired.Further in case the diffraction gratings diffracting a light beam with acertain determined wave length most are applied as mentioned above, thecolor of the image on the focus plane is different only on the part onwhich the diffraction gratings are provided, which is convenient becausethe light measuring range can be visually recognized.

Below the diffraction gratings presenting a self convergentcharacteristics shown in FIG. 10 will be explained. There is a limit forthe dimension of the diffraction gratings presenting a self convergentcharacteristics. The explanation will be made in accordance with FIG.22. Now let the angle between the normal line 81 and the plane 80 onwhich the diffracted light beam advancing from a point E of thediffraction gratings toward the converging point F is incident be θ, θmust be smaller than or equal to sin⁻¹ (n₁ /n.sub. 0) in order that thediffracted light beam can be led out at the end surface 82. Hereby n₀ isthe refraction index of the air, namely 1, while n₁ is that of the glassplate. Let the refraction index of the glass plate be 1.5. θ assumes thevalue 41.8°. In consequence the territory of the diffraction gratings inwhich the diffracted light beam be led out efficiently is limited to atriangle F, G, H with the angle 83.6° at F. The largest radius r of thelight measurement circle with the center 0 of the diffraction element 80is given by a/2 sin θ (r = a/2 sinθ), whereby a is the length of a sideperpendicular to the end surface of the glass plate. Consequently whenfor example, a = 24 mm and α = 41.8, r = 7.8 mm, so that a comparativelynarrow light measuring range nearly such as for the spot lightmeasurement is obtained. However, for the mean light measurement atleast r must be equal to 20 mm (r = 20 mm).

FIG. 23 shows the 12th embodiment of the diffraction element capable ofobtaining a large light measuring range. In this 12th embodiment of thediffraction gratings 90 are shown parallel. The light beam 91 diffractedby means of the diffraction gratings 90 is directed to the reflectingplane 93 opposite to the end surface 92 to be reflected by the plane 93toward the end surface 92. Hereby the reflecting plane 93 present theefficiency for converging the diffracted light beam 91, namely being aconcave mirror. Hereby it is desirable that the center of the curvatureof this concave mirror assume the position F. In this way the diffractedlight beam can be converged on the end surface 92 so as to be directedto the light detecting means 94. FIG. 24 shows the optical path of thediffracted light beam 91 from a point Q of the diffraction gratings 90.As is clear from this drawing, the diffracted light beam 91 reaches thediffraction gratings 90, whereby there takes place a danger of therepeated diffraction. In order to prevent this repeated diffraction itis desirable that a diffraction gratings of volume type with narrowangle selection characteristics for the incident light beam be made useof. Further in case even with this diffraction grating of volume typethe repeated diffraction takes place, the structure of the gratings onthe part on which the repeated diffraction takes place can be varied soas to prevent the repeated diffraction or as is shown in FIG. 25, theinclination β can be given to the reflecting plane 93, so as to changethe incident angle of the second time. Further in case a diffractionelement presenting diffraction gratings with such self convergentcharacteristics is provided in the neighborhood of the focus plane of asingle reflex camera, it is desirable that the end surface at which thelight beam is led out be positioned at a proper place above the image onthe focus plane, namely the place at which the image of the sky isformed.

Below the problems taking place when a plural number of diffractiongratings are overlapped each other and the measures against them will beexplained in accordance with an embodiment. In order to carry out alight measurement over as large an area as possible by means of acompact and thin diffraction element, it is necessary that the anglebetween the normal line and the led out light beam diffracted for thefirst time be chosen large. In such a case, there takes place a problemthat the sensitivity of the beam splitter varies depending upon thepolarization of the incident light beam, which will be explained belowin accordance with FIG. 26.

FIG. 26 shows the case the red light beam R and the blue light beam Breach one diffraction element 100, whereby the phase diffractiongratings 101 of volume type assumes the direction satisfying the Bragg'sdiffraction condition. Let the angle between the first diffracted lightbeam R-1 and the incident light beam be 90° for the red light beam R.The intensity of the first diffracted light beam R-1 varies dependingupon the polarization state of the light beam R. In case the light beamR is of S polarization in FIG. 26, the intensity of the first diffractedlight beam is maximum, while the light beam R is of P polarization theintensity of the first diffracted light beam is zero. This phenomenon issimilar to the case with the polarization angle of Brewster. Inconsequence, for the red light beam of the P polarization the lightmeasurement can not be made by means of this beam splitter. Further thewave length of the blue light beam B is shorter than that of the redlight beam so that the diffraction angle of the blue light beam B isalso smaller and the angle between the first diffracted light beam B-1and the incident light beam B is smaller than 90° in such a manner that,although not so much as in case of the red light beam, the intensity ofthe first diffracted light beam is weaker when the incident light beamis of P polarization than when it is of S polarization and therefore thesensitivity is lowered.

FIG. 27 shows the 13th embodiment for solving the above mentionedproblem. Seen from the side, the beam splitter 100 shown in FIG. 27assumes the construction as is shown in FIGS. 28 and 29. As is clearfrom these drawings, the beam splitter 100 consists of the firstdiffraction element 102 and the second diffraction element, whereby ineach diffraction element 102. 103 a phase diffraction grating 104, 105of volume type is incorporated. Further the angle between the directionof the carriers of these diffraction gratings is 90°. Now let us supposethat the red light beam R₁ be diffracted by the diffraction gratings 104whereby the angle between the incident light beam and the firstdiffracted light beam R1 - 1 be 90°. Further let us suppose that the redlight beam R2 is diffracted by the diffraction gratings 105 whereby theangle between the incident light beam and the first diffracted lightbeam R2 - 1 be 90°. Namely each diffraction gratings is same as thatshown in FIG. 26. They are only arranged at right angle to each other.FIGS. 28 and 29 respectively show a beam splitter seen from the sidemaking a right angle with each other so that the light beam of Ppolarization in FIG. 28 is that of S polarization in FIG. 29, while thelight beam of S polarization in FIG. 28 is that of P polarization inFIG. 29. In consequence when R1 and R2 are of P polarization in FIG. 28the intensity of the light beam R₁ - 1 is zero while R2 is of Spolarization in FIG. 29, so that the intensity of the light beam R2 - 1is maximum. Further when R1 and R2 are of S polarization in FIG. 28 theintensity of the light beam R1 - 1 is maximum while the light beam R2 isof P polarization in FIG. 29 so that the intensity of the light beamR2 - 1 is zero. From the above, it can be understood that the loweringof the sensitivity due to the polarization characteristics can beavoided by measuring the summary of the output signals of the lightdetecting means 106 and 107.

Further, in case as is shown in FIG. 30, the phase diffraction gratingsof volume type presenting carries with different directions are recordedin a doubled way quite the same effect can be expected. Hereby thediffraction gratings recorded in a doubled way can be obtained by meansof the manufacturing method shown in FIG. 9, whereby the photo-sensitivematerial 24 is turned at right angle together with the prisms 21 and 22and exposed much. When the light beam R is of S polarization in FIG. 31,the intensity of the first diffracted light beam R1 - 1 is maximumwhereby the light beam R is of P polarization in FIG. 32 so that theintensity of the first diffracted light beam R2-1 is zero. Further whenthe light beam R is of P polarization in FIG. 31, the intensity of thefirst diffracted light beam R1 - 1 is zero, whereby the light beam R isof S polarization in FIG. 32 so that the intensity of the firstdiffracted light beam R2 - 1 is maximum. At this time, the lowering ofthe sensitivity can also be prevented by measuring the summary of theoutputs of the light detecting means.

So far the case the directions of the carriers of the diffractiongratings make 90° with each other has been explained. Although sucharrangement as mentioned above is optimum, the polarizationcharacteristics can be improved to some extent even when the directionof the carriers of the diffraction gratings make an optional angle witheach other.

What is claimed is:
 1. A device for measuring light incident on an imageforming optical system, comprising:an image forming optical system forforming an image of an object on an image plane with which an opticalpath is formed; a plurality of diffraction lattice elements arranged inthe optical path, each of which being carried by a transparent member,each having constant predetermined size and being substantiallydifferent from each other in area; and a photo detector means to whichdiffraction light from said plurality of diffraction lattice elements isdirected.
 2. A device for measuring light incident on an image formingoptical system, comprising:an image forming optical system for formingan image of an object on an image plane with which an optical path isformed; a plurality of diffraction lattice elements arranged in theoptical path, each of which being carried by a transparent member havingtwo surfaces and an exit surface, and having substantially differentarea with respect to each other, whereby, a portion of light coming fromthe image optical system is diffracted by the diffraction lattice andthe diffraction light is reflected by the surface and directed to theexit surface; and a photo detector means to which light coming from theexit surfaces is directed.